Questions to discuss: I added a shield when designing, and the result is that the effect of not adding a shield is better than that of adding a shield during the test. Why? Does it have anything to do with the shielding cover of the PCB design?
Let's talk about the concept of the shield first. This has to mention EMC control. We know that the three elements of EMC are interference source, interference path and sensitive source. Then the three magic weapons to control EMC are filtering, grounding and shielding, and the shielding cover is a shielding method. There is also the mobile phone board, as the saying goes: the space is not enough, the shielding needs to be combined, that is to say, the mobile phone board needs to add a shielding cover under the high density. Then there is the radio frequency signal, in which case a series of shielded cavities are used. These are the various forms of shields mentioned above.
Then let’s take a look at the role of the shielding cover: simply speaking, there are basically two functions, one is to prevent external radiation interference, and the other is to shield external radiation interference; to be more complicated, it is to prevent external electric fields, magnetic fields or electromagnetic fields from affecting internal equipment. Interference or in order to avoid the influence of the electromagnetic field of the device on the outside world, put the device in a closed or nearly closed metal shell or metal mesh cover.
Based on the above basic knowledge, the answer we get is:
The above problems may be related to the PCB design, but it may not be related.
Specific reasons may be:
First: Are you using the correct size of the shield?
Each RF PCB must be installed in a shielding cavity. The shielding cavity has a large number of resonance frequencies. The resonance frequency is related to the mechanical size of the shielding cavity, and also to the layer structure and medium of the PCB. In RF PCB design, attention should be paid to the lowest resonant frequency. When the operating frequency is close to the lowest resonant frequency, part of the energy will be absorbed, causing the peak value to attenuate, thus affecting the normal operation of the circuit. Therefore, the appropriate size of the shielding cavity should be selected so that the resonant frequency does not fall within the working frequency band of the microstrip circuit. As for the calculation formula, it is left to the hardware engineer. PCB design engineers need to pay attention to the following design points: increase the aspect ratio of the shield, and try to avoid using a square shield. In addition, put some high devices to increase the height of the shield.
Second: Whether the shielding cover is grounded or in good contact with the shielding frame.
If the contact between the shielding cover and the shielding frame on the PCB is not very close, it will sometimes open and sometimes short circuit. This behavior mode is like a switch. As switches are nonlinear components, there will be nonlinear effects, and harmonics are one of the typical nonlinear effects. And any metal, if it is not completely grounded, it is a radiator, so the shielding cover plus the PCB onboard shielding frame, the whole shielding is like a resonant cavity structure, coupling will occur, and the energy will be radiated out, plus the shielding cover and PCB If the switching effect of the on-board shielding frame, the radiation spurious will be stronger, and there will be a risk of exceeding the standard.
Third: Whether the distance between the shield and the device is sufficient.
Due to the close distance between the shield and the RF traces, or the matching components, its parasitic capacitance will affect the impedance, especially for 0402 size components, which are more prone to this phenomenon because of their larger size.
Fourth: Are you using the material of the shield correctly?
Commonly used shielding materials are high-conductivity materials, such as copper plate, copper foil, aluminum plate, aluminum foil, steel plate or metal plating, conductive coating, etc. Electrostatic shielding is mainly used to prevent the influence of electrostatic fields. For example, to protect the interference of small instruments or devices, copper and aluminum compact shells are used as shielding covers. Low-frequency electromagnetic interference, the shell is made of magnetic materials such as iron or beryllium-molybdenum alloy. For the protection of precision electromagnetic measurement interference at low temperature, superconducting material shielding cover and so on are used.
Fifth: The internal radiation spurious of the circuit itself is large.
If the radiation stray inside the shield is too large, after the shield cover is added, the radiation stray cannot radiate outside, and will be coupled into the conduction stray. Therefore, when the radio frequency circuit itself is not well done, the conduction stray without shielding cover is better than that with shielding cover.
Also, what circuits are shields commonly used for?
The purpose of the shield is to isolate, and the circuits that are easy to interfere with each other can be considered isolated:
1) For example, the radio frequency unit and the intermediate frequency unit, the intermediate frequency signal of the receiving channel will cause great interference to the radio frequency signal; on the other hand, the radio frequency signal of the transmitting channel will also cause radiation interference to the intermediate frequency signal.
2) For example, the local oscillator circuit in the radio frequency area should be shielded separately. We know that the local oscillator level itself is high, which will cause greater radiation interference to other units.
3) For example, the digital signal processing circuit is a strong radiation source, and the steep upper and lower edges of the high-speed digital signal will interfere with the analog radio frequency signal.
4) For example, in a cascaded amplifier circuit, the total gain may exceed the spatial isolation from the output to the input, so that one of the oscillation conditions is met, and the circuit may be self-excited.